Here's another thought.

You could divide your trip into three one month bites. Take a month from Seattle to NY; a month down the east side and in Florida (there is so much more than just Disney); and a month working your way back to LA, via all the great national parks.

The only snag I see is, that you could run into campgrounds shutting down for the season in the east. And of course, you need to check if you can get an RV for that long.... and that many miles.

Lifey